1. Field of the Invention
The present invention relates to a fuel cell system and a method of starting a fuel cell system.
2. Description of the Related Art
Fuel cell stacks (also referred to as fuel cells) are known as power sources for vehicles and other electric units. The fuel cell stack includes a plurality of single stacked cells. Further, another fuel cell is known which supplies a humidified oxidizing gas, for example, humidified air to a cathode side to keep an electrolyte membrane of each cell humid. Thus, an amount of water contained in the gas flowing through the fuel cell system becomes large due to water produced by, for example, power generation and humidification. Accordingly, if a temperature of the gas decreases, water contained in the gas becomes condensed. If the fuel cell system is used in winter or at cold regions, the condensed water may freeze in the fuel cell system when a temperature of the fuel cell system becomes lower than an ice point.
Thus, it is desired to develop a technology for preferably operating the fuel cell system at startup of the fuel cell after freezing in accordance with a state of the fuel cell system such as a freezing state and a thawing state (non-freezing state).
Japanese laid-open patent application publication No. 2002-93445 discloses a fuel cell system including a heater for heating the oxidizing gas (air) supplied to the cathode side of the fuel cell, in which the heater is controlled to heat the inside of the fuel cell system when an outside air temperature is subfreezing at the startup.
However, since the fuel cell system judges whether low temperature startup control is required in accordance with the temperature of the system at the startup, when the temperature of the system is high, the low temperature startup control is not done irrespectively of variation of the system temperature during a stop interval. For example, if, after a system temperature reaches a temperature lower than 0° C., the system temperature at the startup is higher than a predetermined temperature, although a part of the fuel cell system is in a freezing state or in an unstable state, the system should start a power generation in the unstable state because the low temperature startup control is not done.